The present invention relates to a drying section for drying a traveling fiber web, particularly a paper web. The drying section is preferably part of a paper manufacturing machine. Various drying sections of this type are known, for example, from U.S. Pat. No. 5,311,672 and from Federal Republic of Germany Patent Publication No. 43 28 554 A1.
From these publications it is known to divide a drying section into several successive drying groups. Any one of the drying groups can be developed either as a single-felt drying group or as a two-felt drying group. In one known variant, all drying groups are developed as single-felt drying groups, for instance in the manner that all cylinders have the felt on their top so that the bottom side of the web always comes into contact with the cylinders.
The foregoing arrangement affords the following advantages: The web to be dried is supported and guided continuously by a support belt ("drying wire" or "felt"), at least within each individual drying group. Unsupported lengths of web are thus avoided so that even with extremely high paper web speeds (on the order of up to 2000 meters/min), fluttering of the web is avoided and the danger of the web tearing is considerably reduced. At the transfer zones between adjacent drying groups, there is the option of either avoiding an unsupported (open draw) section of web or of providing a short unsupported length of web. If, in rare cases, the web should nevertheless tear, the torn web pieces (broke) can be easily removed by the force of gravity, due to the fact that all cylinders are top felted.
For the manufacture of certain types of paper it is disadvantageous to dry the paper web solely from one side. The two sides of the finished web may have slightly different characteristics. This results in a tendency to curl, i.e. the edges of the finished web or the edges of sheets of paper (produced from the web) do not lie flat, but bend downward or upward. The paper generally curls towards the side which was dried indirectly, i.e. the side which was in contact with the felt rather then the dryers.
So-called mixed or composite drying sections are also known in which at least one two-felted drying group is provided downstream of several single-felted drying groups. The final drying of the paper web generally takes place in the two-felt drying group, the two sides of the web alternately contacting the drying cylinders. With this arrangement, for numerous types of paper, the tendency to curl is substantially reduced or even completely eliminated. From several United States patents it is known, in the case of such a two-felt drying group serving for the final drying, to control the supply of thermal energy to an upper row of cylinders independently of the supply of thermal steam energy to the lower cylinders. In this way, the tendency of the final paper to curl is also prevented or reduced.
The known measures described above may be sufficient for the manufacture of numerous types of paper. In many cases, however, it is desired to counteract the tendency of the final paper to curl with even greater certainty.